Sulfuric acid fortification in isoparaffin alkylation



Mrch 2, 1948. A. R. GoLDsBY Er AL 2,437,091

SULFURIC ACID FORTIFICATION IN ISOPARAFFIN ALKYLATION Filed Jan. 25, 1945 Patented Mar. 2, 1948 SULFURIC ACID FORTIFICATION IN ,ISOPARAFFIN'ALKYLATION Arthur R. Goldsby, Beacon, and Claude W. Watson, Eastchester, N. Y., assignors to The Texas Company, New York, N. Y., a corporation of Delaware Application January 23,1945, serial No. 574,137

claims. (o1. 26o-683.4)

l This invention relates to sulfuric acid alkylation of an isoparaln with an alkylating agent, such as an olefin, for the production of gasoline hydrocarbons of high antiknock value suitable for aviation gasoline and motor fuel.

One of the principal objects of the invention i 'manner in order to achieve the foregoing results.

Still another object of the invention is to provide for the continuous fortification of acid recycled in the process with S03 vor fuming H2S04 in accordance with the water content of the recycle acid stream and under conditions to prevent reaction with carbonaceous material in the acid and avoid other deleterious'results.

A further object of the invention is to provide a multistage alkylation system having series flow of isoparaliin and also preferably acid catalyst, and split feed offolefln, Ywith continuous fortification of the recycle acid with fuming acid and return of the resulting fortified acid to the multistage system in a manner to maintain the Water content of the system acid throughout the multiple stages at the low level specified above.

An additional object of the invention isto provide a novel method in continuous sulfuric acid alkylation, with continuous discharge of a portion of the system acid to recovery and recycle of the balance together with makeup and/or fortifying acid, whereby the organic content of the system acid is maintained substantially above that heretofore employed, while concomitantly maintaining the Water content of said system acid substantially below that heretofore obtainable.

Other objects and advantages of the present invention will be` apparent from the following description when taken in connection with the ac companying drawing and appended claims,

In the continuous sulfuric acid isoparaffln-olefin alkylatlon process, it has been specified that vthe titratable acidity of the system acid should be maintained above about 90% for C4 olens and above about 87% for C5 olens, where strong makeup acid of about 98% strength is employed.

However, only recently has it been recognized and on acid life, when that water content builds up in continuous operation. Predrying of the hydrocarbon feed to the alkylation reaction zone has been proposed in an effort to eliminate objectionable water dilution in the system. It has now been discovered that this procedure, while helpful, is not a complete solution of the problem. Water is synthesized in the alkylation process as a result of side4 or secondary reactions which cannot be effectively suppressed, and this serves to build up the water content of the system acid in continuous operation to an objectionable level in conventional alkylation units operating with acid recycle, even where predrying ofthe hydrocarbon feed is employed.

The present invention is predicated on the discovery that the water buildup in the system acid, by virtue of Water synthesized in theralkylation process, should be continuously removed to maintain the water content of the system acid below 4% by weight and preferably below about 1% by weight. This is accomplished in accordance with the present invention by continuously fortifying the recycle acid stream with S03 or fuming H2S04. While the use Vof`l`uming acid as makeup in sulfuric acid alkylation has heretofore been suggested, this has never become commercial practice because of diiculties encountered in its use that were considered inherent and unavoidable. It is generally recognized that the presence of free S03 in the alkylation acid is objectionable and should be avoided.

While the prior suggestions to employ fuming acid as makeup failed to correlate the amount vand strength of the added fuming acid with lthe Water content of the system acid, so that free S03 may have been present in the resulting fortified acid, this is not the sole cause of the diiiiculties previously experienced. The addition of turning acid to a stream of recycle acid containing an appreciable proportion of water liberates substantial heat as a result of reaction and dilution. Consequently, when the addition is carried out on recycle acid at ordinary temperature as it is conventionally returned from the settler, or even at a somewhat reduced temperature due to chilling or evaporative cooling in the reaction zone and/or settler, nevertheless the heat'liberated is suicient to raise the temperature of the fortied acid stream substantially above 40 F. Thishas yresulted in rapid attack of the S03 or fuming acid on the organic material present in the recycle acid, with the formation of a tarry viscous black liquid which is unsatisfactory for the alkylation reaction, necessitating the dumping of the acid so treated.

It has now been discovered that, when the recycle acid stream containing both organic matter and water is prechilled to a temperature substantially below 40 F., such as about 20-30 F., the requiredamount'pfffuming acid'to-react with substantially the .entire Water :content ,of recycle acid containing less than about water by Weight can be added With eiiicient mixing and without raising the temperaturebf rthermixtnre above 40 F.; and at these low temperatureszofi about 40 F. and below, the ruining' acid preferentially reacts with the Waterpfancldoes'.notdeleteriously attack the organic.content-"off` 'thefrecycle acid in the short time-'required .to complete the reaction of free S03 with the Water and thus remove free S03 from the resulting mixture.

In accordance with the present invention, the

recycle acid stream is prechilled, fuminglacid -is added to the prechilledrecycle stream with good agitationto avoid ylocalized".reactionA and=over heating, the Vamount, and fstrengtlr'ofv the fuming acid is controlled fin accordanoefwith theV water montentr of thelrecycle acidstreamto reduce that watercontent to alowivalue ofthe.l order of less Y t-han 2% and preferably 1ess'-than- 1%, suchfas abouti G25-0.75%, while".a'tfthefsame time "the fpresence' of'free S03 'in' the resulting'fortiiied're- Acycle stream is avoided; and zthetemperaturel of prechilling ofl'the recycleacidstream is controlled y so that the heat liberated on'additioniofthe fumfing acid to the recycle acidstreamdoes not Araise #the temperature ofthe mixtureiabove 40F. V'The amount of chillingw :required is dependent? on the 'water content of the recycle'.r acid fand :thefinitial temperature of the-latter. Thefresulting forti'e'd and chilled recycle acid Astream1V is-thenzreturned to'the alkylation reaction zone man amount to `maintain the-water content of the systemsacid *therein below 4%" by'weight and preferably beflow' 1% byweight. `VPreferably, the fortified` and l`-chilled recycle-acid stream isrst thoroughly mixed withisoparain, either feed or recycle'isoparaiiin l or bothparid fthe -resulting `mixture then passed to the falkylation 'reaction zone.

*In continuous operation,v whereY a substantial AAAamount of acidis continuously recycled'in the wsystem; the amountof-fum'ing acidi required to #control the water content may also-sulziply.Y the re- 4Jquiredv makeup I-IzSOi'to-prevent` buildupV of `organic matter in the system-acid above a predeterfm-ined maximum. It is obvious that la more dilute Y*-uming HSOi, such asl one containing 8%. free LS03, will add more H2SO4 per unit amoimt of free S03 than say a` furning acid containing Y20% free "S03, WhilefSOa itself can be employed to control fthe water content, asa 'matter o1V convenience it kis generally preferred vto use fuming acid; and "the strength of the uming acid available in the s upplytanks and feed pipes is selected in accordance with the freezing or congealing point to avoid solidication in the lines at normal atmoszpherictemperatures encountered. For' these reays'onsfa `supply ofV fuming acid v'containing about '1^`20%freefSOs` is*generally-used;andj'if desired, -this-maybe diluted lwithfHzSOi tore'duce the free "SOr content at` or just prior to mixing with the v'recycle'acid stream.

Where the amount vofi-12504 added in the fuming" acid, 'in which the lfree S03 isproportioned in accordance with'the water content of the recycle acid stream, lis insuicient tol supply *the required lmakeup &SO4 tomaintain the'required acid to 'hydrocarbon ratio inthe system, lthen additional acid of low or close to zero water content and devoid of free S03 is also added during continuous operation. For example, it may be necessary to increase the proportion of settled acid discharged to recovery in order to prevent organic buildup in the system acid above a predetermined maxi- 5 muniY as more-particularly described hereinbelow;

and the amount and strength of fuming acid added in accordance with the water content of the recycle acid stream may be such that the rate of 4withdrawal of settled acid from the system 1.0...exceeds;the rate of addition of HzSOi in the fuming acid. This would reduce the acid to hydro- ?fcarbon ratio in the system with continued operation? and additional makeup acid of as near zero water content as is readily available and which is devoid oi' freeSOs is then added to compensate -fortheexcess amount of acid discharged to recovery and thereby maintain the acid to hydror carbon ratio in the system.

Preferably, 995-16070 H2SO4 on an organic- .freeand ash-free basis is-employed as such additional makeup, -where that rstrength of acid is :conveniently and economically available. In such .'case, the said 99.5-100% makeup acid may be added at anyy place'in the system following the settler or evenv directly to the alkylaticn reactor; but preferably this makeup acid is added either 4to the turning acid stream' or to the isobutane recycle stream so as to be thoroughly mixed with the prechilled and fortified recycle acid prior to introduction into the alkylation reactor. Ordinarily, a strength of acid less than 96% H2504 on'an organic-free and ash-free basis will not be employed for makeup, to thereby avoid adding objectionable quantities of water to the system. It'willbe understood that the strength kof the 'makeup' H2SO4 is expressed on an organic-free -a'nd ash-free basis, so that the balance between the expressedstrength of the acid and 100% consists essentially of water.

White acid, as the term implies, is substan- 'tially free of organic content. However, this `white acidmay contain some dissolved mineral matter or inorganic salts, such as lead sulfate, ferrie sulfate, etc. Small amounts of this dissolved mineral matter up to about 3-4% or somewhat higher, expressed as ash content by weight, -can be present in the makeup acid Without deleterious results. Consequently, the titratable acidity of the makeup white acid is expressed on an ash-free basis to show water content, although it will be understood lthat the ash content of such acid is generally small. Black acid on the other `hand contains organic or carbonaceous matter in addition to water and some dissolved mineral matter. Ordinarily, the organic content of strong black acid will run about 1-5% with a water content of about 14% and an ash content of about 0.5-1.5%. As described in greater detail hereinbelow, the organic content of makeup black acid is definitely not objectionable and may even be advantageous in this alkylation reaction, so long as organic buildup in the system acid above a predetermined maximum is avoided. The titratable acidity of makeup black acid is expressed on an "organic-free and ash-free basis to show Water mcontent, since the latter is the critical or objectionable constituent of the makeup acid. Wherever the expressions 99.5-100% H2504 and 9S-99.5% HzSOi are used in the following description and claims as referring to makeup acid, it will be understood that this is on an organicfree and ash-free basis. Consequently, 99.5- v100% `HzSCi means a makeup sulfuric acid having 0.5-0.0% water; and 9S-99.5% -H2SO4 means a makeup sulfuric acid having Li.00.5% water.

n the other hand, where titratable acidity of the system acid, that is, theacidpresent in the alkylation system including the reactor and settler, is mentioned; in the following` description and claims,v it will be understood thatthis means the measured acidity orH2SO4 by titration which will be less than 100% by the combined amounts of water,V organic material and mineral matter which are present therein.

Preferably, the makeup white or black acid has a titratable acidity on an organic-free and ashfree basis of at least 98% to avoid water dilution; and the ash content of said make-up acid generally runs about 0.0-l.5%',y although it may be somewhat higher without objection. The organic content of makeup black acid/is maintained bellow so that organic buildup in the system acid above about 20% is prevented. Where the 'available makeup acid contains in excess of 1/a% water, thus having a titratable acidity on an organic-free and ash-free basis of 96-99.5% H2SO4 and preferably 98-99.5% H2SO4, this makeup acid of appreciable water content is added to the re- ',cycle acid streamA in advance ofthe chiller. And

the amount and strength of added fuming acid is then proportioned to compensate for the water introduced with the makeup acid as well as the water present in the recycle acid. It will be understood that the fuming acid is prepared from White acid substantially free from organic content, to avoid attack of the S03 on carbonaceous Imaterial of black acid.

The present invention is more particularly illustrated in the attached drawing disclosing pre- .'ferred embodiments thereof and wherein Fig. l is a diagrammatic view of a single stage present invention; and

Fig. 2 is a diagrammatic view of a multistage alkylation unit having serial-flow of isoparanin and preferably acid, and split feed of olefin, and incorporating the features of the present invention.

vlteferring to Fig. 1v, feed'isobutane from line I 0,

'afterv passing through a suitable drier I I, is intro- ;duced by valve-controlled line I2 into the recycle line'I3 containing the recycle acid and recycle lolei'in feed, which may be a C4 renery cracked gas fraction, is passed by line I6 through a'suitable drier' I1 and thence into the alkylation re- 6.. equipment and then-tor stabilizing and fractionating towers in conventional manner. The settled acid layer is continuously removed by bottom line 23, a position thereof being discharged to recovery by valve-controlled line 24, and a substantial proportion, usually a major proportion, diverted into the valve controlled acid recycle line 25. Preferably,y thesettler 2| is operated with a. minimum acid level indicated at 26 by sufliciently rapid withdrawal of the settled acid from settler 2| to maintain the level of ilat acid in the settler at the lowest point consistent with proper operation of the usual automatic controls.

',Ihe recycle acid stream passes to a suitable chiller 28 where the temperature is reduced substantially below 40 F., ordinarily to around 25- 30 F. The chilled recycle acid then flows through line 29 into which is introduced by line 30 S03 or fum-ing H2S04. For a convenience in description, the expressions ffuming acid or fuming H2S04 are hereinafter employed in the description and claims to include S03 as well as H2804 in which S03 has been absorbed to produce fuming acid. As stated above, the fortifying S03 is most conveniently applied in the form of S03 absorbed in sulfuric acid; and the fuming acid strength is generally selected to provide for low freezing or congealing point. Consequently, the material supplied by line 30 will generally be fumng acid containing about 20% free S03. The amount and strength of theV fuming acid added by line 30 is controlled in accordance with the water content of the recycle acid stream passing through line 29. This is conveniently accomplished by regularly sampling acid Withdrawn at 24 and making a water determination thereon. It will be understood that line 29 will be equipped with a suitable ilow meter, so that the amount of free S03required to react with and reducethe H2O content of the recycle acid can readily be computed from the jvelocity fof Iiow of the recycle acid through line 2-9v-and the mentioned water determinationcn that acid. v -4 The resultingfmixturecis immediately passed throughl a suitable mixer `32,r suchl asv a baiiied compartment providing turbulent iiow, to insure Vthat the reaction between the free S03 and the f culation of refrigerant or other cooling medium actor-I4. If desired, a portion or all of the isobutaneA feed from line I2 can beV by-passed by valvehcontrolled line I3 for mixing with the olen feed inline I6 to thereby adjust the isobutane to voleflnr'atio, such as tol produce in excess of about a.l lzl'molar ratio, in the olefin feed line. It will be understood that the oleiin feed supplied to line I6 haspr'eferably been caustic-washed and then water-washed. While predrying of the hydrocarborigfeedas disclosed is the preferred procedure,

Vit is -to be understood that this is not essential in duly high, such yasto render; it diiiicult to ,pump

the 'stream and secure proper turbulent flow `and vmixing of" thatastream with the fumng acid.

The.. presence yof a-swbstantialgsoluble organic content in` the,v acid recycle stream lowers the freezing or congealing point; and ordinarilyprechilling of that. stream aslowas about 20 F. can be accomplishedv without diiiiculty.y However, in those .cases ywhere the equilibrium conditions of the continuous system would encounter viscosity diilculty on prechilling of the' acid recycle stream to the required low temperature to properly absorb Ythe heat orreaction -andrdi- 7. lution upon addition -at one pointofl the total amount vof fuming: acidi-,o reducetlie Water content to the desired'lowfigure, then the `furning acid is added `to the recycle yacid. stream in stages with intercoolers between the stages. Thus, the recycle 'acid is first prechilled to a temperature above that at which the viscosity increases unduly, a `portion of. the fuming acid is then added and thoroughly Vmixed with the stream and the latter is Aagain refrigerated, as by the use of a separate mixer followed by an intercooler or by the use of a combined mixer and intercooler, additional fuming acid is then added, and the process repeated in the required number of stages to reduce the water content to the desired low figure While at the same time preventing local overheating and maintaining the temperature of the' mixture below 40 F. The Water content of the resulting mixture discharged by line 33 is less than 2% by Weight and preferably less than 1%, and may be of the orderl of about 1Ar%%, and usually about 1/2%. `'This is to insure that no free S03 remains in the recycle acidintroduced into the reactor Iii, there- Vby avoiding prolonged contact and resulting reaction of free S03 With the organic content of the systemracid. At the same time, the water content of the recycle acid is there-by continuously reduced to such a lowgure that the system'acid 'in reactor I4 is maintained at a Water content bel-W 4% by Weight and preferably'below 1%. Thechilling of the-recycle acid stream in chiller 28, or in chiller 28 -coupled With mixer cooling or one or more intercoolers as described above, is controlled so that the temperature of the mixed recycle acid and fuming acid does not rise above 40 F. This can be accomplished in any suitable manner, such as by regulating the supply of refrigerant to each chiller, or by the use of a valve-controlled by-pass around each chiller. I

The resulting fortified'and chilled acid recycle stream is mixed in line 33 vwith isobutane recycle from line 35. It Will be understood that the hydrocarbon phase removedvby line 22, after being caustic-washed and water-washed, is then passed to the conventional stabilizer or debutanizer, where normal butane and lighter is re- 'moved overhead. The resulting olf-gases are 'event where the HzSOifcontainin'g.absorbed S03 `supplied by line 30 is insufficient to Provide the :required makeup acid to compensate for that 'discharged to recovery by line 24, additional acid which is devoid of freeSOs and which contains as low a Water content 'as practicable, is introduced into the system by either valve-controlled line 38 or valve-controlled line 39. Where 99.5100% H2504 is available for makeup, then it is added by line 38 through either valve-controlled branch line 40 into the fuming acid line 30, or through valve-controlled branch line 4l into the isobutane recyclelnev35. Where the thus passed through .chiller 2'8;v andthe amount of fuming acidadded by line 30 is then proportioned to compensate for the Watercontent of the combined recycle acid andmakeup acid, so that the resulting fortifled acid `of line 33 has a low Water content of the order specified above.v

Fig. 2 discloses a five-stage concurrent flow unit With split feed of olefin of thecharacter disclosed and claimed in the copending application of Arthur R. Goldsby and: Claude W. Watson Serial No. 574,136, filed of` even date herewith. As shown, feed isobutane from .line 45 passes through drier 46 and thence is introduced by valve-controlled linef4'l into thevrecycle line 48 containing fortified recycle acidmixed with recycle isobutane. This mixture is introduced into the firststage 5D of theY multistage alkylationunit having the .fivereactors Ell-54 .of the eccentric rotor type, although other conventional types o'f contactors can be employed. Olefm4 feed from line 55 passes through Adrier 56 and thenceis split by the valve-'controlled branch lines. 51--6I between the reactors --54 0f the multistage unit. If desired, a. portion or all of the isobutane feed from line 45 can be introduced by the valvecontrolled branch line 62 into the-olefin feed line to raise the isobutane to olen ratio of this olefin feed. Y

Each of thebranch lines 516I constitutes a manifold supplying a large number of pipes or nozzles indicated at 63 which serve to introduce the olefin feed into each reactor by multipoint olefin addition. It Will be understoodthat this means the introduction ofthe olefin intothe agitated or' circulated stream of hydrocarbon or catalyst in finelyv divided form and at a large number of spaced points extending throughout a substantial portionl of the volume of the mechanically agitated mix in each reactor. By combining thefeature of 'ne subdivision of the olefin with the finite spacing of the substantial number of points of olefin introduction, so as to prevent any substantial accumulation or buildup of olefin in any increment of the acid in the reaction mixture, as disclosed and claimed in our copendng application Serial No. 476,896 filed February 24, 1943, local overheating and alkyl ester buildup in the acid catalyst are prevented, and objectionable 'side reactions are minimized.

It will be noted that in the preferred arrangement. the bulk of the isobutane and acid are introduced by line 48 into the rst stage 50 to flow serially through thev system,.along with alkylate produced in each stage. by'means of the connecting lines E4-6L This means that the highest isobutane concentration in the hydrocarbon phase of the reaction-mix will bei found in stage 1; and the isobutane concentration is progressively reduced from stage to stage throughout the system. The amount of olefin injected by multipoint Vaddition in each stage is preferably regulated in accordance With the'isobutane concentration in that stage whereby the amount is progressively reduced vfrom theV rst to thelast stage of the system. Inthis-manner, a superior quality of alkylate can be produced in high capacity with a lower overall isobutane requirement for the system,V thereby enabling the amount ofisobutane recycleto be vmaterially.re-

' duced with resultanteconomy in stage 54 by line 69 to settler 10, where stratification occurs into a hydrocarbon layer, whichis removed by line 1I to the above-described neutralizing, washing and fractionating equipment,

and an acid layerindicatedat 12 which is removed by bottom line 13;' A regulated portion of the .acid from line 13 vis discharged to recovery by line 14, and the balance. is recycled by line .15'

through chiller 16 to line 11. Fuming acid is introduced at this point by line 18 in accordance with the water content of the recycle acid stream, and the mixture ows through mixerL 19 where lreaction .of free S03 with the water content and dilution to ahomogeneous composition are completed. y

The resulting fortified and chilled stream then passes by line 80, together with isobutane recycle from line 8l, through mixer 82 to line 48 for return to the first stage 50 of the alkylation system. As shown, any makeup 99.5100% H2804 is added by line 84 to line 48 to be introduced into the first stage of thel system. This is of particular application to amodied operation .in which a portion of the fortified recycle acid, either unmixed or mixed with a portion of the isobutane recycle, is split -between the stages as described hereinbelovv. Where substantially all of the recycle acid and isobutane recycle are introduced into the first stage, the strong makeup acid can of course be introduced into'the line atV any point as described abovein connection with Fig. 1. Weaker makeup acid, such as 9699;5% HzSOi., is introduced by line 85 into the recycle line in advance` of chiller 16 as described in connection with Fig. 1. 'l

As an alternative procedure, a portion of th fortied'recycle acid can be Vpassed by valvecontrolled line 81 to manifold 88 and distributed by valve-controlled branchv line 89-92 between stages 2-5 of the alkylation system, the balance passing together with isobutane recycle from line 8l through mixer 82 and thence'by line`48 to the irst stage 50. This provides jfor independent control of the catalyst to hydrocarbon ratio in the individual stages, andalso permits independent control of the water content of the system acid in each stage. Thus, the catalyst to hydrocarbon ratio may be progressively varied or reduced from' stage to stage.V .For example; the reaction mix in the first stage operating with the highest acid loading rate A(volume of olefin 'added per volume of catalyst per hour)'may contain about (S5-'15% acid by volume; and' this ratio may be progressively reduced in succeeding stages until in the Afinal stage; operating with the lowest acid loading rate, the acid content of the reaction niix maybe approximately 50% by'volume.

' In addition, the various" stages may be operated with catalyst holdup, such as by the provision of reactors in'which partial settling is permitted in the upper portion orspace from which the emulsion stream is overowed or discharged to the next stagejjin series,' ;whereby hydrocarbonenriched emulsion thus'passes from stage to Stage and@ substantial ptopelftmnfgihe catalyst Y i0 is retained in each individual stage. In this manner, a catalyst activity gradient may be maintained throughout the system, varying from a maximum activity in the iirst stage to a minimum activity in the final stage. In this case, any required makeup 99.5-100% H2S04 is added by line 84 topass into the -Iirst stage and thus iiow serially through the system at a slower rate than the flow of hydrocarbons through the system. The amount of fortified recycle acid introduced into subsequent stages from line 81 is then regulated primarily to control the water content inthe vsubsequent stages, the` organic contentof the system Iacid progressively building up until acid discharged from the final stage is largely spent, whereby the bulk of the settled acid removed by line 13 can bev discharged by line 1li-to recovery. In that case, only sufficient recycle acid is returnedby line 15 to provide for control of the water content as heretofore described- As a further alternative, a portion of the forti- Alied` recycle acid mixed with isobutane recycle can be passed byl valve-controlled line 94 to manifold 88, land this mixture distributed in independently controlled amounts between stages 2&5 by branch linesl BSI-92, Vthe balance being passed byline 48 to the iirst stage 50. In this manner, the yacid to hydrocarbonrratio in the various stages can be independently controlled and varied'and the water content of the system acid also controlled,lw hile the insobutane concentration canwbefmaintained higher in the subsequent stagesv than where, substantially all of the isobutanerecycle is introduced into the first stage forvserial iiow through the system. This is of particular application where sufficient isobutane isV available to maintain a high isobutane concentration in all stages.

In both Figs, 1 andl conventional operating conditions for sulfuric acid alkylation other than those specified above vcan be employed. For example, overall isobutane: oleiinmol feed ratios of about 3:1 to 10:1 and preferably at least 5:,1 may be used, together with an acid to hydrocarbon volume, ratio inthe reactor-of around 1:1, a temperature lof, 30-60" F. and preferably ever, the present invention v,is particularly useful in connection with the employment of higher acidv to hydrocarbon ratios in the reaction mix, for example, a ratioy of L5-90% by volume and preferably about -79% by volume. It is towbe understood that the lmaximum acid to hydrocarbon ratio which'can be tolerated for best results is a function yof the mixing efliciency, and therefore dependsl upon the particular type vof reactor used. The upper limit for best results with the Stratco reactor and the pump and time tank reactor appears to be about by volume of acid; but higher ratios can be employed with other types ofk reactors having higher mixing efficiency. j.,

While the present invention has been particularly described in connection with the alkylation of isobutane Awith a butylene, it is to be understood that itis applicable to the alkylation of 13 REFERENCES CITED The following references are of record in the le of this patent:

UNITED STATES PATENTS Numbef Name Date Frey May 21, 1935 Number 

